1. Field of the Invention
The present invention relates to an electrically driven power assisting device such as a power steering device for a vehicle.
2. Description of Related Art
Recently, as a power steering for a vehicle, an electrically driven power steering device using an electric motor in place of a hydraulic motor is used and as the torque of a motor in the same rotational direction as the following one is required to be transmitted according to the clockwise or counterclockwise rotation of the steering of a vehicle, the prior art in which the armature of a motor is rotated according to the clockwise or counterclockwise rotation of a steering is disclosed in Japanese unexamined patent application publication No. H6-316270.
However, this device has a problem that the flow of electric current to the motor is required to be switched so as to rotate the motor in the same direction as the clockwise or counterclockwise rotation of a steering wheel, measures to suppress armature reaction and spark noise generation are required because excessive current flows when the flow of current is switched, further a plurality of diodes and transistors for switching the flow direction of current are required.
In Japanese unexamined patent application publication No. H7-61362, a movable body is straight spline-coupled to either one of a stub shaft or a pinion shaft and is helical spline-coupled to the other shaft. A first or second clutch mechanism is operated by the movable body moved by the rotation of the stub shaft. The unidirectional rotation of a motor is reversed by a driving bevel gear and a pair of driven bevel gears and transmitted to the pinion shaft.
This device has a problem that the life and the precision of a torsion bar are deteriorated because torque for moving the movable body via the helical spline coupling by twisting the torsion bar provided between the stub shaft and the pinion shaft through the rotation of the motor is not required to be reversed and transmitting the rotation of the motor is directly applied to the torsion bar via the helical spline coupling.
In Japanese unexamined patent publication No. 6-293266, since a movable member is moved by the steering force of a driver to couple a clutch mechanism, in consideration of that the clutch mechanism is expected to be smoothly switched to switch the direction of the steering assistance force without giving unpleasant feeling to the driver when the rotation of a steering wheel is suddenly switched to the right or to the left, for example, if the clutch mechanism is an engagement type clutch such as a ratchet type clutch, the vibration caused when the clutch mechanism is coupled or released is directly transmitted to the steering wheel, which is not desirable. In the meantime, since the power steering device is given the steering assistance force by an electric motor such that steering can be performed by the steering force of about 5 Nm, the extent of movement of the movable member which is produced by the steering force of the driver can not be set too large. Therefore, since the pressing force for engaging the clutch plate can not be increased only by the steering force of the driver, it is necessary to use material having a large coefficient of friction or to increase the contact area by enlarging the outer diameter of the clutch plate so as to obtain the required pressing force. However, it is natural that the material having the large coefficient of friction is disadvantageous in the wear of the friction contact area and in durability. Moreover, if the outer diameter of the clutch plate is enlarged so as to increase the contact area, it is natural that the enlarged device is enlarged, which results in a problem.
In Japanese unexamined patent publication No. 62-273165, an electromagnetic clutch which is operated based on the steering torque of a driver can be used and hence the magnetic force generated by the exciting coil of the electromagnetic clutch can produce the pressing force of the clutch plate (engaging force). Therefore, the clutch corresponding to the output of the electric motor can be supplied by increasing a current supplied to the exciting coil or the number of turns of the exciting coil. However, if it is required that the engaging force of the clutch is completely supplied only by the attracting force of the electromagnetic clutch, the size of the electromagnetic clutch becomes large.